Christopher H. Gammons

Geochemistry of the rare-earth elements and uranium in the acidic Berkeley Pit lake, Butte, Montana

Abstract Filtered (0.45 μm) and nonfiltered concentrations of rare-earth elements (REE), U, Zr, Th, Ba, Sc, and Y were measured as a function of depth in the Berkeley Pit lake, a large acidic mining lake in Butte, MT. The REE show very little variation with depth, apart from a slight concentration near the surface, presumably due to evaporation. The REE profiles of the pit waters show a depletion in light REE when normalized against NASC and the host Butte Quartz Monzonite (BQM), and a possible enrichment in middle REE vs. heavy REE when normalized against BQM. All of the REE partitioned weakly into secondary ferric precipitates formed by aging of deep Berkeley Pit water. The measured distribution coefficients increased across the lanthanide series from La (Kd=7.7) to Nd (Kd=10.3), and then decreased steadily to Lu (Kd=1.4). Under the conditions of the Berkeley Pit lake, the aqueous speciation of REE and U is dominated by sulfate complexes. Because the stability constants of REE–sulfate complexes show very little variation across the lanthanide series, the observed trends in Kd cannot be explained by aqueous complexation. Despite high concentrations of dissolved REE (e.g., 1.1 mg/l Ce) and U (0.85 mg/l), saturation indices for all solid phases were strongly negative, due to the low pH of the Berkeley Pit lake (2.3–2.6). The mobility of REE and U is more likely constrained by adsorption or co-precipitation with strengite, jarosite, schwertmannite, or other secondary minerals forming in the lake.

The aqueous geochemistry of REE.: Part 8: Solubility of ytterbium oxalate and the stability of Yb(III)–oxalate complexes in water at 25°C to 80°C

Abstract The solubility of ytterbium oxalate (Yb 2 Ox 3 · x H 2 O) in aqueous solution was measured as a function of temperature (40°C, 60°C and 80°C), ionic strength (0.05, 0.1 and 0.2 m NaCl), and free oxalate concentration (10 −7 to 10 −2 molal). At high oxalate and/or NaCl concentration, the simple Yb–oxalate salt transforms into a double salt with the stoichiometry NaYb(Ox) 2 · y H 2 O. Our solubility data were used to obtain smoothed equilibrium constants ( I =0, P=SVP) for the solubility products of both solid phases, and for the stepwise and cumulative association complexes of the aqueous Yb(III)–oxalate complexes, between 25°C and 100°C. Whereas the solubility of the simple Yb–oxalate salt changes little in this temperature range, the Na–Yb oxalate salt becomes less stable and significantly more soluble with an increase in temperature. The ytterbium oxalate complexes are strong, and may predominate over inorganic Yb(III) complexes (e.g., carbonate, chloride, sulfate, and phosphate) at geologically realistic ligand concentrations. Our results are relevant to researchers interested in the transport of ytterbium and other rare earth elements (REE) in near-surface waters and sedimentary basin fluids, and may also have engineering applications with regards to the precipitation of oxalate salts as a means of removing actinides and radioactive REE from aqueous nuclear wastes.

Influence of copper recovery on the water quality of the acidic Berkeley Pit lake, Montana, U.S.A.

The Berkeley Pit lake in Butte, Montana, formed by flooding of an open-pit copper mine, is one of the worlds largest accumulations of acidic, metal-rich water. Between 2003 and 2012, approximately 2 × 10(11) L of pit water, representing 1.3 lake volumes, were pumped from the bottom of the lake to a copper recovery plant, where dissolved Cu(2+) was precipitated on scrap iron, releasing Fe(2+) back to solution and thence back to the pit. Artificial mixing caused by this continuous pumping changed the lake from a meromictic to holomictic state, induced oxidation of dissolved Fe(2+), and caused subsequent precipitation of more than 2 × 10(8) kg of secondary ferric compounds, mainly schwertmannite and jarosite, which settled to the bottom of the lake. A large mass of As, P, and sulfate was also lost from solution. These unforeseen changes in chemistry resulted in a roughly 25-30% reduction in the lakes calculated and measured total acidity, which represents a significant potential savings in the cost of lime treatment, which is not expected to commence until 2023. Future monitoring is needed to verify that schwertmannite and jarosite in the pit sediment do not convert to goethite, a process which would release stored acidity back to the water column.

Acid Rivers and Lakes at Caviahue-Copahue Volcano as Potential Terrestrial Analogues for Aqueous Paleo-Environments on Mars

Mars carries primary rock with patchy occurrences of sulfates and sheet silicates. Both Mg- and Fe- sulfates have been documented, the former being rather uncommon on Earth. To what extent can a natural acidic river system on Earth be a terrestrial analog for early Mars environments? Copahue volcano (Argentina) has an active acid hydrothermal system that has precipitated a suite of minerals in its hydrothermal reservoir (silica, anhydrite, alunite, jarosite). Leakage from this subterranean system through hot springs and into the crater lake have formed a strongly acidified watershed (Rio Agrio), which precipitates a host of minerals during cooling and dilution downstream. A suite of more than 100 minerals has been found and conditions for precipitation of the main phases are simulated with speciation/saturation routines. The lower part of the watershed (Lake Caviahue and the Lower Rio Agrio) have abundant deposits of ferricrete since 2003: hydrous ferric oxides and schwertmannite occur, their precipitation being mediated by Fe-oxidizing bacteria and photochemical processes. Further downstream, at greater degrees of dilution, hydrous aluminum oxides and sulfates form and create ‘alcretes’ lining the river bed. The watershed carries among others jarosite, hematite, anhydrite, gypsum and silica minerals and the origin of all these minerals could be modeled through cooling/dilution of the primary hot spring fluids. Single evolution (acidification through capture of volcanic gases, water rock interaction to acquire the dissolved cations) through cooling of the primary fluids could explain most of the Fe-bearing minerals, but to precipitate Mg-sulfates, evaporation and renewed interaction with olivine-rich rocks is needed to saturate some common Mg-sulfates (e.g., epsomite). The schwertmannite beds formed through processes involving Fe-oxidizing bacteria, which may be significant if this mineral was common on Mars in the past. Photochemical processes on Mars are commonly discussed in terms of photo-oxidation of Fe, but photo-reduction may be a common process as well, as was found to be the case in the Rio Agrio watershed. A model of waters acidified by the capture of S-rich volcanic gases that have reacted with basaltic rocks, and then evaporated or were neutralized by higher alkalinity surface fluids may explain the origin of the sulfate mineral suites on Mars quite well.

Biogeochemical and microbial seasonal dynamics between water column and sediment processes in a productive mountain lake: Georgetown Lake, MT, USA

Montana Institute on Ecosystems (0739054); National Science Foundation (1338040); MSU Undergraduate Scholars Program

GEOCHEMISTRY AND HYDROGEOLOGY OF ACID MINE DRAINAGE IN THE GREAT FALLS-LEWISTOWN COAL FIELD, MONTANA 1

The Great Falls-Lewistown Coal Field (GFLCF) in central Montana contains over 400 abandoned underground coal mines, many of which are discharging acidic mine water with serious environmental consequences. Completely submerged areas of the mines are not strongly acidic, whereas water quality quickly deteriorates in portions that are only partially flooded. In general, the pH of the GLFLC mine waters can decrease or increase after discharging to the surface, depending on the initial ratio of acidity, mainly as dissolved ferrous iron, to alkalinity, mainly as bicarbonate ion. Although the chemistry of many of the adit discharges is nearly constant with time, large diurnal and seasonal changes in the quality of down-gradient waters have been observed. Decreases in concentrations of dissolved Fe and Zn during the day at one location are explained by an increase in the rate of Fe 2+ oxidation and precipitation of ferric hydroxide as the water warms. The precipitation of ferric hydroxide resulted in sorption of Zn 2+ , a reaction that was also thermodynamically favored by an increase in water temperature and pH during the afternoon. Historical efforts to passively treat acid mine drainage in the GFLCF using wetlands, limestone channels, and anoxic drains have been unsuccessful, due to the harsh climate, high metal concentrations, and acidity loads of the mine waters. Alternative mitigation concepts are currently being evaluated that focus on source control rather than treatment.

A comparison of filtered vs. unfiltered metal concentrations in treatment wetlands

Filtered vs. unfiltered metals analyses are compared from two demonstration wetlands built by ARCO in Butte, Montana. The Wetlands Demonstration Project 1 (WDP1) facility was an anaerobic, subsurface flow wetland, whereas the Colorado Tailings (CT) facility was a lime-added, aerobic system. At both sites, a significant fraction of each metal of concern (Cu, Cd, Zn, Fe, and Mn) existed in particulate form in some parts of the treatment system. The anaerobic WDP1 wetland removed dissolved metals to very low levels, but had mixed success in filtering out fine-grained sulfide precipitates of Cu, Cd and Zn. The CT wetland showed better capacity to remove particulate metals. Based on these two case studies, the importance of obtaining both filtered and unfiltered (total recoverable) samples at treatment wetlands is stressed.